Beams of charged particles are used in medical application for cancer treatment when cancerous cells are localised within a specific region of the patient's body. Recent techniques allow for a precise extraction of a given dose of radiation in a target volume of the patient. Charged particle therapy systems usually comprise a particle accelerator for producing a beam of charged particles and beam extraction means. The means for extracting the ion beam may comprise beam deflecting means such as magnets, energy degraders, etc.
A charged particle therapy system is for example described in document EP 2446718. This document generally relates to a pulsed beam particle accelerator which can be used for particle radiation therapy. More particular, a device and method are provided to control the number of particles within a beam pulse to irradiate an irradiation spot in a target volume. The particle accelerator comprises means for varying the number of particles within each beam pulse of the pulsed ion beam from a minimum value to a maximum value as a function of the value of a beam control parameter. For each particle irradiation the required number of particles for each beam pulse is controlled by defining a value for the beam control parameter based on calibration data.
In order to irradiate a specific irradiation spot in a target volume, beam control elements such as scanning magnets and/or an energy degrader are generally used. The setting of the beam control elements may take place in between subsequent particle pulses of the beam, for example. The operation of a synchrocyclotron for particle radiation therapy however also requires an assessment of the status of the beam control elements before allowing the departure of a bunch of particles from the accelerator. This assessment must be done within the time available between bunches, and for some beam control elements, the assessment is done preferably as late as possible, in order to allow the setting of the beam control elements a maximum of time. On the other hand, the assessment must be finished fast enough so that cancellation of the beam is still possible. Due to deviations from the theoretical frequency cycle of the synchrocyclotron, for example because of imperfections in the rotating capacitor used for generating the cycle, presently known systems do not allow to ensure that all assessment steps are concluded in time, because the beam control elements have no way of taking the cycle deviations into account. This results in ineffective system operation, for example involving the extraction of a bunch despite a negative assessment of the status of a number of beam control elements.